1. Field of the Invention
The present invention relates to a multi-column fluorescent display tube (hereinafter referred to as a fluorescent display tube) suitable for use in small-sized electronic equipment such as an electronic clock of a digital display type.
2. Description of the Prior Art
Generally, a fluorescent display tube has fluorescent material coated elements which emit light when electrons emitted from a cathode come into collision therewith, and therefore can obtain a very clear display. For this reason, the fluorescent display tube is widely used for small-sized electronic desk calculators and various digital-display electronic apparatus. In addition, a drive circuit according to a dynamic drive system is generally provided for actuating the fluorescent display tube. In such a drive circuit, provision of only one decoder is sufficient for forming input signals to be applied to segment anodes of each of multi-column pattern display sections so that the segments anodes may selectively display a desired pattern such as a figure; in short, only one decoder is sufficient for actuating the multi-column pattern display sections. Therefore, the circuit can be made simple in constitution, and the number of large scale integrations (LSI) incorporated in the circuit and that of lead-in terminal wires of the fluorescent display tube can be decreased. Thus the circuit of this kind is widely used for multi-column electronic desk calculators. The fluorescent display tube used in combination with a drive circuit of a dynamic drive system usually carries out the switching of column selection by a control electrode, and therefore it is in the form of a triode provided with a mesh-shaped control electrode. However, the above-mentioned drive circuit according to a dynamic drive system has disadvantages in that, when it actuates the fluorescent display tube, pulse current flows in the fluorescent display tube and the circuit to thereby cause noises, and therefore it can not be applied to the display of an electronic clock or the like incorporated in an acoustic or image-displaying electronic equipment such as a clock radio which must avoid access to noise sources.
Accordingly, a fluorescent display tube to be incorporated in an acoustic or image-displaying electronic equipment which must avoid access to noise sources should be preferably made so that it can be used in cooperation with a drive circuit according to a so-called static drive system which has no pulse power source acting as a noise source. In the case of a drive circuit according to the static drive system, current is made to flow between the segment electrodes and the cathodes corresponding to a column selected according to a pattern such as a figure to be fluorescently displayed, and therefore, a control electrode for column selection is not necessary. Accordingly, a fluorescent display tube for use with this circuit is in the form of a diode. In the case of the fluorescent display tube in the form of a diode, the upper surface of a base plate around segment anodes tends to be unevenly electrified due to electrons and the like emitted from the cathode. In addition, when the upper surface of the base plate is electrified, the electric field and electron current in a space between the cathodes and the segment anodes where electrons emitted from the cathodes are passed through becomes disturbed resulting in an uneven luminescence of the fluorescent material layer mounted on each segment anode and, what is worse, the fluorescent material layer on a segment anode which is desired to luminesce becomes unable to emit light. Furthermore, an external electric field changes the electrification conditions around the segment anodes and thereby exerts an influence on the electron current flowing between the cathode and the anodes with the consequent result that a normal luminescence of the fluorescent material layer can not be achieved. For the reasons mentioned above, a fluorescent display tube incorporated in a drive circuit according to a static drive system has been heretofore provided with a mesh-shaped control electrode between the cathode and a group of anodes so that the electron current flowing between the cathode and the anodes may not be influenced by the electrification conditions and external electric field as mentioned above.